1. Field of the Invention
The present invention relates to a method for estimating radiated emission level from an arbitrary EUT (Equipment under test) on the turn table at OATS (Open Area Test Site). More particular, the present invention relates to a method for estimating radiated emission level from an EUT at an arbitrary position on the turn table at OATS which obtains correlation between GTEM cell and OATS in order to simulate electromagnetic wave radiation emission more accurate than GTEM (Giga-hertz Transverse Electromagnetic) cell, one in the middle of replacing facilities of OATS, a test facility for evaluating radiation emission during a test of electromagnetic interference(EMI).
2. Discription of the Prior Art
1. The existing correlation algorithm
The existing correlation algorithm using GTEM cell was in detail explained in a paper P. Wilson, D, Hansen and D. Koenigstein, "Simulating open area test site emission measurements based on data obtained in a novel broadband TEM cell", in Proc. IEEE Nat. Symp. on Electromagn. Compat., Denver, Colo., May 1989, pp. 171-177 and "Method and apparatus for improved correlation of electromagnetic emission test data," please make reference to Application No. PCT/US 93/07556 filed on '93, 8, 11, Patent No. WO94/04933. The latter is an improved algorithm which connects a part searching a maximum value of voltage which an EUT in GTEM cell radiates at a front step of 3-input correlation algorithm to increase the accuracy of the former, 3-input correlation algorithm. Simply, it will be explained as follows.
First, algorithm of Wilson supposes that the phase of each component of dipole is identical for modelling the EUT as a dipole. The algorithm can be divided into two algorithms which are the correlation of 12-measurements, 12-inputs, and the correlation of 3-measurements, 3-inputs. The former can compute the electromagnetic wave radiation when we should know the respective components of the dipole illustrating the EUT, namely, P.sub.x, P.sub.y, P.sub.z of the electric dipole and M.sub.x, M.sub.y, M.sub.z of the magnetic dipole for expecting the radiation quantity. It appears to reflect the dipole component variously since the computing equation is complicated compared to the latter, the computing value can be a large error due to supposing that the dipole component is in-phase actually. The latter computes a radiation electric field with only a total radiation power from the test matter. The total radiation power requires only three transfer powers from the EUT in GTEM cell. There is the total radiation power achieved in the dipole moment amplitude value of 6 components as follows [Equation 1]. EQU P.sub.0 =10k.sub.0.sup.2 {P.sub.x'.sup.2 +P.sub.y'.sup.2 +P.sub.z'.sup.2 +k.sub.o.sup.2 (M.sub.x'.sup.2 +M.sub.y'.sup.2 +M.sub.z'.sup.2)} [Equation 1 ]
The three transfer powers referenced above are as follows [Equation 2]. EQU P.sub.1 =P.sub.y'.sup.2 +k.sub.0.sup.2 M.sub.x'.sup.2 [Equation 2] EQU P.sub.2 =P.sub.z'.sup.2 +k.sub.0.sup.2 M.sub.y'.sup.2 EQU P.sub.3 =P.sub.x'.sup.2 +k.sub.0.sup.2 M.sub.z'.sup.2
Further, these refers to the power of X, Y, Z. When a small electric dipole exists in a free space, the electric field from the dipole in the far-field zone ##EQU1##
the total radiation power to this dipole ##EQU2##
Accordingly, ##EQU3##
Herein, based on that the total radiation power from the electric and magnetic dipole in arbitrary amplitude and directions is equal to the total radiation power from the electrically small electric dipole in z-direction. That is, supposing that the total radiation power expected from the GTEM cell is the radiation power from a single short dipole, in the case that one dipole moment is dominant, this shows an excellent model for the radiation of EUT. However, in the case that two or three moments are dominant, this model appears to be the worst case. When positioning this dipole on the grounded surface, measuring a horizontal electric field, the maximum couple happens when the dipole is positioned in horizontal, in the case of a vertical electric field, the maximum value will be received when the dipole is positioned vertically. The electric fields of two cases are as follows [Equation 3a] and [Equation 3b]. ##EQU4## ##EQU5##
The 3-input correlation algorithm is used as most GTEM cell correlation algorithm used recently because of obtaining very simply radiation electric field. However, there may be critical errors in specific cases of dipole model.
Osburn algorithm is summarized into two embodiments so that firstly, the EUT is replaced with an actual dipole, E field equal to that from at the respective frequency can be measured, which is provided to apply on EMC standard regarding the power applied to the dipole as the computing value.
Supposition: the dipole, electric model of the EUT is supposed that all components are in-phase.
The voltage measurements of 12 times: 6-faces all measurements of a regular hexahedron wrapping the EUT under two polarizations.
After grasping the position (face & polarization) of the EUT emitting the maximum radiation quantity at the respective frequency from the 12 times measurement, the face & polarization of the EUT at that time are selected as the reference position, the maximum voltage is called X voltage.
Y and Z voltages relative to X voltage are obtained to have the input value of 3-input correlation algorithm.
To compute E field data, these three voltages are used to 3-input correlation algorithm of Wilson.
After computing E field, to compute the power provided to the dipole so that the radiation of identical E field occurs. This power is the value to indirectly compare with the standard limiting value. That is, the first embodiment becomes 12-measurement, 3-input correlation algorithm.
The second embodiment is to obtain the E field data to directly compare with the limiting value of the standard. This doesn't suppose that the EUT for all frequencies can be modelled by the dipole with gain not greater than the dipole. When this supposition is incorrect, the gain of the EUT is computed and used instead of the gain of radiation source in the 3-input correlation algorithm.
Voltage measurements of 12 times: being identical with first embodiment.
Reference Position: the same as the first embodiment.
There is obtained Y and Z voltages for 3-input correlation algorithm of Wilson.
Grasp whether the supposition of the dipole gain at the respective concern frequency is suitable or not. If it is suitable, there is used the supposition relative to the dipole gain in the correlation algorithm such as the first embodiment.
In the case of the frequency whose the supposition is not suitable, the additional measurement is performed at about 45.degree. position from the reference face in the same polarization and at about 45.degree. position from the reference face in another polarization. These measurements are used to obtain the evaluated value of the vertical and horizontal beam width. This gain evaluation replaces the gain value of the dipole in the correlation algorithm.
It is used to compute E field data, in the case that the dipole supposition is not achieved. The second embodiment becomes 16-measurement, 3-input correlation algorithm.
2. The problems of the existing embodiment relative to the correlation algorithm of GTEM cell and OATS.
Because 3-input correlation algorithm and John D. M. Osburn algorithm adding a routine finding a maximum transfer voltage in the front step basically suppose the dipole phase as the in-phase, they can have the error corresponded to it. Accordingly, if considering all phases of the dipole at power X, Y, Z, they are expressed as [Equation 4] as follows. EQU P.sub.1 =P.sub.y'.sup.2 +k.sub.0.sup.2 M.sub.x'.sup.2 -2k.sub.0 P.sub.y' M.sub.x' sin(.phi..sub.py -.phi..sub.mx) [Equation 4] EQU P.sub.2 =P.sub.z'.sup.2 +k.sub.0.sup.2 M.sub.y'.sup.2 -2k.sub.0 P.sub.z' M.sub.y' sin(.phi..sub.pz -.phi..sub.my) EQU P.sub.3 =P.sub.x'.sup.2 +k.sub.0.sup.2 M.sub.z'.sup.2 -2k.sub.0 P.sub.x' M.sub.z' sin(.phi..sub.px -.phi..sub.mz)
Accordingly, the total power has the error by [Equation 5] comparison [Equation 1] with [Equation 4]. EQU 20k.sub.0.sup.3 P.sub.y M.sub.x sin(.phi..sub.py -.phi..sub.mx)+P.sub.z M.sub.y sin(.phi..sub.px -.phi..sub.mz)+P.sub.x M.sub.y sin(.phi..sub.px -.phi..sub.mz) [Equation 5]
Furthermore, even though the total radiation power is accurate, this is a radiation from the electric and magnetic dipole with any direction, the computed value for the actual EUT can have the error in the most case that the EUT is not characterized by the vertical and horizontal electric dipole since this power in the 3-input correlation algorithm is supposed as the radiation quantity from a single short electric dipole.